Over the last decade considerable research has been directed at developing flexible integrated electronic systems capable of supporting a new class of flexible electronic devices. Interest in the field of flexible electronics arises out of a number of advantages promised by this technology over conventional single crystalline silicon based electronic devices. For example, the capability to conform to bent and flexed orientations without fracturing allows flexible electronic devices to be configured in a wide range of useful device geometries, such as bent orientations characterized by a high radius of curvature, not possible with brittle conventional single crystalline silicon based electronic devices. In addition, flexible electronic devices are expected to be more robust with respect to mechanical deformation and shearing relative to comparable conventional single crystalline silicon devices. Moreover, fabrication pathways available for flexible electronic devices using solution processable component materials, polymer-based substrates and/or low temperature, non-clean room processing conditions may enable a high speed, low cost fabrication platform for patterning these devices on large substrate areas.
Progress in the field of flexible electronics is expected to play a critical role in a number of important emerging technologies. For example, advances in flexible electronics are anticipated to enable a range of low cost, large area macroelectronic devices, such as flexible sensor arrays, electronic paper, wearable electronic devices, and large area flexible active matrix displays. In addition, development of flexible integrated electronic systems and processing methods is also expected to significantly impact a number of other important technologies including micro- and nano-fluidics, sensors and smart skins, radio-frequency identification systems, information storage, and micro- and nanoelectromechanical systems. The success of these applications of flexible electronics technology depends strongly, however, on the continued development of new materials, device configurations and commercially feasible fabrication pathways for making integrated electronic circuits and devices exhibiting good electronic, mechanical and optical properties in flexed, deformed and bent conformations.
Functioning flexible semiconductor based electronic devices having amorphous silicon, organic or hybrid organic-inorganic semiconductors have been available since the mid 1990's, [E.g. Garnier, F., Hajlaoui, R., Yassar, A. and Srivastava, P., Science, Vol. 265, pgs 1684-1686], but exhibit significant limitations in their electronic performance. For example, thin film transistors having amorphous silicon, organic or hybrid organic-inorganic semiconductors typically exhibit field effect mobilities approximately three orders of magnitude less than comparable single crystalline silicon based devices. Higher performing flexible electronic devices based on polycrystalline silicon thin films or solution processable nanoscale materials, such as nanowires, nanoribbons, nanoparticles and carbon nanotubes, have been recently demonstrated. However, commercially viable processing platforms capable of exploiting these new technologies for the manufacture of high performance macroelectronic products are yet to be developed. As a result of these limitations, flexible electronic devices are currently limited to a narrow class of electronic devices, such as switching elements for active matrix flat panel displays with non-emissive pixels and light emitting diodes, not requiring high electronic performance.
To overcome the current limitations in flexible electronics new processing methods, materials and device configurations are needed for integrating a wide range of materials, including high quality semiconductor materials, into functional devices on conformable and mechanically robust substrates. Materials and processing methods compatible with device assembly on polymer-based substrates, such as polyimide, polycarbonate or Mylar, are particularly attractive given the robustness, mechanical strength and ability to undergo deformation without fracture of these materials. Despite these benefits, conventional polymer-based substrates are not without limitations relevant to developing high performance flexible electronic devices. First, conventional methods for processing high quality inorganic semiconductor components, such as single crystalline silicon or germanium semiconductors, typically employ thin film growth at temperatures (>1000 degrees Celsius) that significantly exceed the melting or decomposition temperatures of most polymeric materials. Second, although most polymer-based materials are bendable, many of them have a significant rigidity and, therefore, exhibit resistance to changes in conformation once they have been cast and cured. As a result, while conventional polymer-based substrates may be cast into a wide range of shapes and configurations, many of these materials are not capable of readily adapting to changes in conformation after they have been cured. Finally, flexible electronic circuits on conventional polymer-based substrates are susceptible to permanent deformation or delamination when bent beyond a maximum bending radius.
Another approach to providing truly conformable substrates for flexible electronics is integration of electronic devices and circuits with textile materials, such as flexible fabrics. Many textiles are capable of assuming a wide variety of shapes and accommodating substantial deformation and movement without damage or significant degradation. A principle advantage of this approach to flexible electronics is that electrical and mechanical integration of flexible electrical components with textiles, such as flexible fabric substrates, provides an effective means of minimizing strains and stresses generated upon deformation. Electronic devices based on large area flexible textiles, therefore, have great potential for providing extremely versatile devices capable of changing device conformations and geometries while maintaining good device performance. Effective integration of electronic components and flexible textiles is expected to enable a new class of “smart fabrics” having far reaching applications for sensing temperature, pressure and strain, wearable computing, wireless communications and networking, and bio-sensing.
International Patent Publication No. WO 01/30123 describes flexible electronic devices consisting of a plurality of flexible conductive threads interconnecting electronic components that are stitched or woven into a flexible fabric substrate. A number of single and multilayer fabric and electronic component geometries are provided that are alleged to preserve the flexible character of the fiber substrate. While this publication describes integrated fiber and electronic component configurations allegedly providing large area, highly conformable integrated electronic circuits, this approach to flexible electronics is subject to significant limitations. First, as components are integrated with the flexible fiber substrate via conventional sowing or weaving techniques, the approach is not readily adaptable to integration of small (<10 microns) nano- or micron-scale electronic components. Second, the approach appears to be limited to integration of flexible conductive threads and leads, and thus is not amenable to integration of device components comprising brittle materials, such as high quality inorganic semiconductors, ceramics and glasses. Finally, it is not clear from the description in the patent publication that the device configurations disclosed provide an effective means of reducing stresses and strains established on the electrical components themselves upon deformation of the flexible fabric. This structural limitation may substantially reduce the extent and number of conformations available to the electronic devices disclosed in this reference.
It will be appreciated from the foregoing that there is currently a need in the art for processing methods and device configurations for fabricating flexible integrated electronic circuits, devices and systems. Flexible electronic devices are needed that are capable of exhibiting good electrical, optical and mechanical properties in deformed or flexed configurations. In addition, processing methods for making flexible electronic devices are need that are capable of integrating diverse materials having a range of dimensions with truly flexible substrates capable of conforming to a wide range of shapes and orientations, such as flexible fiber substrates. Finally, high throughput, low cost processing methods for making large area flexible electronic devices are needed to enable a wide range of revolutionary flexible electronic devices